Development of a thermo-kinetic model for plutonium diffusion in the mixed oxide (U,Pu)O₂

The uranium and plutonium mixed oxide fuel (U, Pu)O₂±ₓ is used in both LWR and SFR reactors. During its synthesis process by sintering, the conditions have to be optimized to get appropriate plutonium content, oxygen stoichiometry and homogeneity. During reactor operation, the fuel pellet experiences high thermal radial gradient resulting in redistribution of plutonium affecting the fuel behavior. In order to develop a better understanding and prediction of the thermo-kinetic properties of mixed oxide (U, Pu)O₂±ₓ for both sintering synthesis and in-pile fuel behavior, this work focuses on modelling the plutonium diffusion based on the reliable CALPHAD thermodynamic model for mixed oxide fuel. We first analyzed the available literature plutonium diffusion data in (U, Pu)O₂±ₓ mixed oxide. The scarcity and inconsistency of the experimental data was observed. This lack of data was then complimented by the use of cBΩ model for the estimation of plutonium diffusion in PuO₂. Based on our selection of experimental data and the cBΩ results, a DICTRA based diffusion model was developed for plutonium diffusion in mixed oxide fuel. This model is able to compute the plutonium diffusion coefficient at any temperature, plutonium composition and oxygen stoichiometry.